Tangential cutting inserts, also known as on-edge, or lay down, cutting inserts, are oriented in an insert holder in such a manner that during a cutting operation on a workpiece the cutting forces are directed along a major (thicker) dimension of the cutting insert. An advantage of such an arrangement being that the cutting insert can withstand greater cutting forces than when oriented in such a manner that the cutting forces are directed along a minor (thinner) dimension of the cutting insert. Another advantage of such an arrangement is that with the minor dimension directed perpendicular to the cutting forces it is possible to manoeuvre the cutting insert between obstacles close to the workpiece.
For turning a stepped square shoulder on a workpiece, a cutting tool assembly requires a cutting insert with an acute operative insert cutting corner, a tool back clearance angle along its inoperative cutting edge and an obtuse entering angle along its operative cutting edge. Such an entering angle enables an outwardly directed feed out movement to square out a shoulder, in particular, an outwardly directed radial feed out movement in the case of external axial turning operations and an outwardly directed axial feed out movement in the case of radial turning operations.
In view of these restrictions, cutting inserts for turning stepped square shoulders are usually either rhomboidal or triangular; thereby having respectively, two or three indexable insert cutting corners for single-sided cutting inserts. Such cutting inserts are, for example, as illustrated and described in U.S. Pat. No. 4,632,608, each insert cutting corner being formed as a protruding nose portion at the junction between centrally depressed insert sides. The cutting inserts are preferably double sided so as to be respectively formed with four or six indexable insert cutting corners.
With a view to increasing the number of cutting corners, a fully indexable non-tangential cutting insert is described in U.S. Pat. No. 6,074,137. The cutting insert comprises four substantially concave side edges extending between substantially square opposing upper and lower surfaces. Adjacent side edges meet at a cutting corner having an angle in the range of about 83°±5°. Although the cutting insert is substantially square and although it offers eight cutting corners, its depth of cut is limited. In fact, the maximal depth of cut is limited to less than the length of a side of an imaginary square, in which the insert is inscribed, in a top view of the insert. Furthermore, it is not a tangential cutting insert.
FIGS. 1 and 2, show a cutting tool 20 with a tangentially seated cutting insert 22 for both axial and radial turning operations, also known as longitudinal and face turning operations. The cutting insert 22 is oriented with relief angles γ1 and γ2 for radial and axial turning operations, respectively. The cutting insert 22 has one operative cutting corner 24, a first trailing non-operative cutting corner 26 during axial turning operations and a second trailing non-operative cutting corner 28 during radial turning operations. Major and minor cutting edges 30, 32 extend between the operative cutting corner 24 and non-operative cutting corners 28, 26.
FIG. 3 is an illustrative drawing showing the cutting tool 20 during either radial or axial turning operations of a workpiece 33. Dashed lines 34 show an ideal square shoulder and the dash-dot line 35 is an imaginary extension of the worked face 36 of the workpiece 33. As can be seen, for a radial turning operation, the second trailing non-operative cutting corner 28 and a portion of the major cutting edge 30 are oriented such that they “extend beyond” the imaginary extension 35 of the worked face 36 and would engage the workpiece 33 if an attempt were made to increase the depth of cut beyond a depth of cut, d, where the dashed line intersects the major cutting edge 30. Thus, the depth of cut is limited during radial turning of a square shoulder. For axial turning in the configuration shown in FIG. 3, the depth of cut is also limited to d. Any increase in the depth of cut would lead to a non-square shoulder. Similarly, the insert could be configured with an orientation such that for an axial turning operation, the first trailing non-operative cutting corner 26 and a portion of the minor cutting edge 32 are disposed such that they have a limited depth of cut. Likewise, the insert could be configured with an orientation so that it has a limited depth of cut for both axial and radial turning operations due both to the first trailing non-operative cutting corner 26 and a portion of the minor cutting edge 32 and also to the second trailing non-operative cutting corner 28 and a portion of the major cutting edge 30.